Literally no idea how they work? Not super common, although a paper on drug targets in Nature gives this list of drugs where they can't even begin to classify what sort of entity they act on.

But many other drugs are still incompletely understood, or only have very speculative mechanisms of actions. For example, we know that SSRI antidepressants increase levels of serotonin in the brain, but we're not really sure why that should treat depression (the hypothesis that depression is an imbalance of neurotransmitters was originally an ad hoc hypothesis explaining why these sorts of drugs seemed to treat it, although we've learned a lot since then).

Paracetamol (aka acetaminophen aka Tylenol) is actually pretty mysterious and still the subject of a lot of study, as are anaesthetics (which is too bad, because if we understood how anaesthetics worked, we'd have a promising lead in figuring out what consciousness is). Most psychiatric medications range somewhere from "incompletely understood" to "might as well be witchcraft", and lot of neurologic ones like some of the antiepileptics aren't much better.

I was surprised to learn recently that a lot of drug discovery is now being done by brute force. For example, they discovered new cystic fibrosis drug ivacaftor by dosing lung cells in solution with more or less every known organic chemical until one of them caused the chlorine concentration of the solution to change, which indicated that it had somehow solved the error in cellular chlorine transport that causes CF.